A cylinder injection engine configured to directly inject a fuel into a cylinder has been known. In the cylinder injection engine, the degree of freedom of injection timing, that is, the degree of freedom of an air-fuel mixture production is improved as compared with a conventional port injection engine. For example, during catalyst early warm-up at the time of cold start of an engine, a fuel injection is performed during an intake stroke and immediately before ignition (a compression stroke or an expansion stroke) to produce a rich (air excess ratio λ<1.0) air-fuel mixture in the vicinity of an ignition plug, and further an ignition timing is greatly retarded than a normal ignition timing. This makes it possible to raise an exhaust temperature and accelerate the warm-up of a catalyst.
In the cylinder injection engine, a rich portion is locally produced in a combustion chamber, and a particulate matter (PM) is produced due to the occurrence of oxygen deficiency in the rich portion, and the adherence of the fuel onto a cylinder wall surface or a piston upper surface.
For example, when the rich air-fuel mixture is produced in the vicinity of the ignition plug, a PM emission amount is increased due to the oxygen deficiency. For that reason, it is desirable that the fuel injection amount immediately before the ignition is reduced in a range where combustion stability can be maintained. However, a reduction in the fuel injection amount immediately before the ignition leads to an increase in the injection amount during the intake stroke, and in that situation, the fuel adhered to the cylinder wall surface or the piston upper surface increases. Therefore, it is difficult to sufficiently reduce the PM.
In the engine disclosed in Patent Literature 1, the fuel injection is divided in three or more times, and implemented to thereby produce a stratified air-fuel mixture within the combustion chamber. In performing the fuel injection for each combustion cycle, a lean air-fuel mixture higher in local air-fuel ratio (air excess ratio) than 1.0 is produced in the combustion chamber due to a first injection, and thereafter the air-fuel mixture equal to or smaller than 1.0 in the local air-fuel ratio is produced in the combustion chamber due to a second injection. Further, a third injection for producing the ignitable air-fuel mixture locally rich in a region of the ignition plug is performed immediately before the ignition timing.
However, in both of the second injection and the third injection subsequent to the first injection, the air-fuel mixture of the rich air-fuel ratio is produced, and an air-fuel mixture ratio of the rich air-fuel ratio naturally becomes higher in the combustion chamber. In that case, there arises such a problem that the emission amount of the PM is rapidly increased on the border of a theoretical air-fuel ratio. On the other hand, from the viewpoint of ensuring a combustion stability, it is desirable to properly provide a rich air-fuel mixture in the vicinity of the ignition plug.